In recent years surface light emitting apparatuses have been used as the backlight of liquid crystal display, the surface light emitting apparatus having such a constitution as light emitted by a light emitting diode enters a plate-shaped optical guide through a side face thereof and emerges from a principal surface of the optical guide in surface emitting operation.
In the surface light emitting apparatus, light emitted by the light emitting diode is introduced into a plate-shaped optical guide through a side face thereof and is emitted through a predetermined light emission observing surface by making use of reflection in the optical guide. The light emitting diode used as the light source is typically mounted in plurality on a substrate whereon electrical conductors are wired to supply power, with the light emitting surface of the light emitting diode disposed so as to oppose the side face of the optical guide.
Moreover, in the recent trend toward higher luminance of the light emitting diode, it is made possible to reduce the number of light emitting diodes used. In the small-sized backlight used in the light crystal display of a mobile telephone, in particular, it has become possible to supply the amount of light required by the backlight by means of a single light emitting diode.
In the surface light emitting apparatus disclosed in patent documents 1 through 3, for example, light is introduced through a corner of the optical guide, instead of the side face of the optical guide as in the prior art, in consideration of the light distribution characteristic of the light emitting diode and the ease of light emitted by the light emitting diode to spread in the optical guide.
When the number of light emitting diodes is reduced, it becomes more important to spread the light coming from the light source uniformly over a surface. For example, in the surface light emitting apparatus disclosed in Patent Documents 1 to 3 that use the optical guide where light is introduced through a corner, an effective light emitting region used in observing the light emission has generally rectangular shape. This configuration makes it necessary to provide surface light emission so that uniform luminance is obtained at the four corners of the rectangle.
The corner of the optical guide through which light from the light emitting diode is introduced is nearest to the light emitting surface of the light emitting diode, and directly receives the light from the light emitting diode. Accordingly, it is necessary to cause the three corners other than this corner to illuminate evenly. FIG. 6 is an enlarged top view of a corner of an optical guide of a surface light emitting apparatus where light is introduced through an end face located in a corner of the optical guide. FIG. 13 is a perspective view of the corner of the optical guide.
Incident light cannot be spread evenly in every direction simply by forming a light introducing surface by chamfering the corner flat as shown in FIG. 6 and FIG. 13. Sufficient light does not reach, in particular, a region located at a position offset from the straight propagation path of light that has entered the light introducing surface at right angles.
An optical guide disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-357714, for example, has notches formed in the light introducing surface so as to spread light coming from the light emitting diode evenly in every direction within the optical guide. FIG. 5 is a top view of the notches formed in the corner of the optical guide. FIG. 12 is a top view of the notches shown in FIG. 5, viewed in the direction of the light emerging surface. The notches have the shape of triangular prism formed from side faces that include equal sides of the cross section having the shape of isosceles triangle, when viewed from the light emerging surface of the optical guide.
When the light emerging surface is formed in square shape, light can be spread to some extent by forming the notches in the corner.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 10-223021    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 2000-260217    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2001-357714